After a well has been partly or totally drilled, various open hole or cased hole logs are made in the well. One or more different types of logging tools may be incorporated on a common logging cable. Several logging tools utilize radioactive sources to implement measurements in an open or cased hole. The radioactive source is typically an encapsulated selected isotope. The isotopes are selected for a number of factors, one typical factor being to provide a particular radiation emission. Moreover, an isotope which has a relatively long half life provides essentially a constant source so that calibration of the equipment is fixed by virtue of the fact that the radiation level is essentially constant. A very common source material is Cs.sup.137 having a half life of 30.23 years.
The relatively long half life Cs.sup.137 isotope is advantageous in several regards, one mentioned and another being the energy of its gamma ray emissions. However, one distinct disadvantage of this isotope and others of significant long half life is the difficulty that arises when the logging equipment becomes stuck in the well borehole. Assume for descriptive purposes that a tool including a Cs.sup.137 isotope radiation source is stuck and the tool can not be retrieved. One remedial possibility is drilling through the stuck logging tool. This is not desirable because it regrettably converts the logging tool into metal shavings which include shreds of the radioactive isotope. Since the half life is 30.23 years, the shredded materials will remain significantly radioactive for a long time, thereby preventing drilling through the stuck logging tool. If that situation arises, it is often necessary to re-enter the well and deviate the well above the stuck radioactive logging tool, or to completely abandon the well, both costly alternatives.
The method and apparatus of the present disclosure overcomes difficulties associated with Cs.sup.137 radiation sources. The present disclosure sets forth a radioactive source which is a liquid enclosed within a container surrounded by appropriate shielding material wherein the container can be selectively emptied of the liquid radioactive material. The radioactive isotope is thus used in liquid form, secured within a cylinder and pressurized by a motor driven piston. An outlet passage cooperative with a check valve system is included to evacuate the chamber. In the event the tool can not be retrieved, the liquid incorporating the radioactive isotope is controllably forced from the tool, and then purged into the borehole in the vicinity of the tool. Additional fluid can then be introduced at the well head to flush the radioactive isotope liquid into permeable zones below the stuck tool, or to the very bottom of the well, or alternatively, through perforations of the well into selected sands. In any event, the radioactive material is evacuated from the area of the stuck tool and is transferred to a location where it poses no problem to subsequent drilling through the stuck logging tool and subsequent production work.
One type of source employed could be a liquid form of conventional isotope such as Cs.sup.137, however a preferred alternative would be to use a radioactive isotope material having a relatively short half life. As noted above, Cs.sup.137 half life is measured in years. The preferred alternative material for the present invention is a short half life radioactive material. Appropriate isotopes include I.sup.131 which has a half life of 8.07 days or Au.sup.198 which has a half life of 2.693 days. If either of these isotopes is used, the radioactive isotope material in the logging tool may quickly decay to a nonhazardous state or can be dispersed into zones adjacent the borehole or into circulating fluid in the well. Alternatively, these radioactive materials can be in liquid form and purged in the described fashion to the lower portions of the well so that the radioactive materials offer no impediment to drilling through the stuck logging tool. If the short half life radioactive were short enough, it might even be possible to allow either removal option i.e., permitting the tool to either be drilled through with the radioactive material in place in the logging tool or, alternatively, purged to a remote safe location in the borehole.
One of the features of the present disclosure is the use of a fluid isotope which is received in the apparatus within a closed chamber which isotope can be controllably evacuated by means of a motorized pump. The evacuation apparatus includes a flow line from the apparatus including a check valve, and an outlet passage from the tool so that radioactive source material can be timely evacuated. This can be used in conjunction with apparatus at the well head for introducing additional fluid flow into the well borehole to purge the radioactive source material to the bottom regions of the well borehole for disposal. In conjunction with alternative relatively short half life radioactive source materials, this apparatus provides a logging tool (e.g., a density logging tool) radioactive source to provide the required radiation intensity level for operation of the logging tool(s) and which tool, in the event of being stuck, can either be drilled through with safety or alternatively, which source will quickly decay to sufficient low levels that safety hazards are not created. The preferred apparatus includes control means for selectively operating a motor for forcing the radioactive source material out of the tool for downhole purging.